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May 2011

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From:
Baines Alan - Slough <[log in to unmask]>
Reply To:
The Pharmaceutical Microbiology Forum Email List <[log in to unmask]>
Date:
Tue, 17 May 2011 15:24:47 +0200
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Dear Anders,

When looking at PPC recoveries, it always pays to remember that the recovery % is a prediction based on a calculation that is dependant on the standard curve in use. As you will be aware, in the case of a linear regression, the calculation is based off a straight line fit, whereas the actual curve may well be'bowed' to some degree. Indeed the shape of the standard curve often means that the highest degree of over-prediction is seen in the mid-point of the curve, right where the value of the PPC is being predicted. This effect is most easily seen with the PPC recoveries when testing water samples. The PPCs for water samples are often >100% because of this issue.

Another confirmation of this effect can aslo be seen when using the recombinant Factor C assay, as this assay, unlike the kinetic chromogenic or turbidimetric assays has a positive slope. Use of linear regression here, generates results where most water PPC recoveries fall below 100%, usually between 90-100%. In either case, it is possible to reduce the effect of the curve shape on the PPC recovery but using a polynomial based standard curve fit, such as 'Powercurve'.

On top of the above is the normal variability that is seen with the LAL test, so it is always wise to avoid setting a target that is statistically difficult to achieve, such as inserting in an SOP that PPC recoveries should say, be within 95-105% as this would lead to unnecessary repeat testing and reporting of a 'deviation' that is not real.

So to answer your query, it is definitely the case, that provided the recovery is well within the 50-200% allowed PPC recovery percentages, consistency is more important than trying to hit the magical 100% recovery mark and if you pick a suitable dilution from a screening assay when the PPC recovery has leveled off, you will get much more consistent results, after the three batch validation of that dilution, with your routine testing.

Regards,


Alan Baines
Global Sales Director
Rapid Testing Solutions
Lonza Wokingham Ltd
Registered Office:
228 Bath Road, Slough SL1 4DX
United Kingdom
Tel: 0808 234 9788
Fax: 0808 234 9778
Direct: +44 1753 504335
Mobile +44 (0)7887 596196
[log in to unmask]
www.lonza.com


-----Original Message-----
From: The Pharmaceutical Microbiology Forum Email List [mailto:[log in to unmask]] On Behalf Of ADET (Anders Thorn)
Sent: Monday 16 May 2011 10:10
To: [log in to unmask]
Subject: [PMFLIST] Interference in the LAL test

Dear Forum,

When you make a screening of different dilutions of a sample for LAL test that cause inhibition of the assay the recovery of the positive product (%PPC recovery) increase with the dilution. Often you will see that the curve (% PPC as function of the dilution) level off when the %PPC recovery is above 100 % (for instance at around 130 % which corresponds to the %PPC recovery of LAL reagent water or WFI tested in your assay) and not when the %PPC recovery is exactly 100 %.

When is a solution considered free for interference in the LAL test? Is it when you dilute your sample so you get a %PPC recovery at around 100 % or is it when you dilute your sample to a degree where the curve (% PPC as function of the dilution) level off (for instance at around 130 %)?

Thanks and best regards,
Anders

_______________________

Anders Thorn

Novo Nordisk A/S

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The PMFList (http://microbiol.org/PMFList_info.htm) is operated from
The Microbiology Network (http://microbiol.org) and supported by
our sponsors (http://microbiol.org/sponsor.htm) as a service to
the scientific community.

Please take a second to visit our sponsors' web sites and say thank you for their support of this service.

Accugenix - http://www.accugenix.com

American Type Culture Collection - http://www.atcc.org

BD Diagnostic Systems - http://www.bd.com/ds/

Biolog - http://www.biolog.com

Biomerieux - http://industry.biomerieux-usa.com

Biotest - http://www.BiotestUSA.com/micro

MicroBioLogics, Inc. - http://www.microbiologics.com

MODA Technology Partners: now a LONZA company http://lonza.com/moda

Pall - http://www.pall.com

NovaTek International - http://www.ntint.com

Rapid Micro Biosystems - http://www.rapidmicrobio.com  (formerly GPS)

Veltek Associates, Inc - http://www.sterile.com

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The nature of this service is to provide a medium for communication.  The specific statements and endorsements of individuals participating in the discussions are not necessarily those of The Microbiology Network, Inc., the PMF, or the sponsors of the list.

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